The Ports are successfully opened in firewall. I can connect to the server but I get, once every few files I download from the internet, this error in filezilla:

primary connection and data connection certificates don't match

After a lot of reading, I found an answer that says : We need to ensure that a valid SSL certificate is set both at the global and the individual FTP site levels. It finally depends upon the FTP client whether to expect the same certificate or different ones for primary and data connections.